1. Field
The invention relates generally to wireless communication, and more specifically to header compression in broadcast and multicast services.
2. Background
Broadcast, or multicast services, refers a communication system used to transmit information from a transmitter to multiple receivers or users. Examples of broadcast, or point-to-multipoint communication systems include dispatch systems, such as used by police, trucking companies, and taxi companies where a central dispatcher broadcast signals to one or more vehicles. The signal may be directed to a specific vehicle or to all vehicles simultaneously.
As mobile radio networks have become commonplace, such as cellular telephone networks, customers have begun to desire to receive broadcast of video, and multimedia using Internet Protocol (IP) over a wireless communication link. For example, customers desire to be able to receive streaming video, such as television broadcast, on their cell phone or other portable wireless communication device. Other examples of the type of data that customers desire to receive with their wireless communication device include multimedia broadcast and Internet access.
A typical wireless communication channel has limited bandwidth and may experience significant error rates. Various techniques for transmitting messages according to broadcast and multicast services (BCMCS) have been developed. In general, these techniques include formatting the message data into packets with a header that includes information about the data within the packet. In BCMCS communications, a content provider, or content server, generates a data stream to be broadcast to multiple receivers, or users. The data stream is converted to data packets that include headers, comprising a BCMCS data stream that is then broadcast to multiple communication devices simultaneously. Due to the scarcity of radio resources, such as limited bandwidth, the packet data headers are usually compressed before being broadcast.
Header compression refers to the technique of minimizing the necessary bandwidth usage for information carried in headers of various packets; examples include RTP/UDP/IP and TCP/IP packets. Header compression takes advantage of the redundancies existing between consecutive packets within a given packet stream. For compressing the packet headers, a compressor may reside within a wireless network infrastructure such as in a Packet Data Serving Node (PDSN). The compressor receives an incoming BCMCS data stream from a content server, compresses the BCMCS data stream and then forwards the data stream with the compressed headers to a radio network. The compressed packets are then transmitted over a radio link broadcast channel to the designated wireless communication devices. The receiving wireless communication devices include a decompressor that receives the compressed packets as input and regenerates uncompressed packets to reconstruct the original data stream.
Most header compression schemes make use of the redundancies between consecutive packets by maintaining state information (context) at both the compressor and the decompressor. The static context information is sent only initially, while dynamic context is sent either as uncompressed values in dynamic context update headers to complete the context information, or as differences from packet to packet, usually encoded using efficient encoding algorithms. In order for the decompressor to regenerate the uncompressed packets correctly, the context in the decompressor needs to be synchronized to the context used by the compressor during compression. Techniques that have been developed to maintain synchronization of the context between the decompressor and compressor include the Robust Header Compression (ROHC) technique developed by the ROHC Working Group of the Internet Engineering Task Force, [see, for example the standards and drafts at the Internet URL www.ietf.org/rfc/rfc3095.txt?number=3095], incorporated in its entirety herein. While the ROHC protocol improves header compression for broadcast services, such as BCMCS, due to the limited bandwidth of the wireless communication channel further improvements in header compression are desirable.
There is therefore a need in the art for improving header compression techniques for broadcast and multicast services in a wireless communication system.